Method and apparatus for sorting articles such as letters

ABSTRACT

System and method for sorting articles, particularly letter mail, in such fashion that articles having a common characteristic, e.g., letters with the same address, are delivered as containerized group containing at least a predetermined threshold number of the articles. Applied to systems of the type wherein a classifying apparatus identifies each article and delivers the articles to individual article retaining stations while retaining cognizance of the characteristics and locations of the articles, the invention employs receptacles which are advanced past the retaining stations, articles having a common characteristic being discharged into the same receptacle, means being provided whereby any receptacle in which less than a predetermined number of articles have been accumulated can be transferred to storage and later retrieved and advanced again past the article retaining station for addition of more articles having the same characteristic.

United States @atent i-ieni 1 Septll, 1973 [5 METHOD AND XPPARX'ifiS'FKR" 57 ABSTRACT ARTICLES SUCH AS System and method for sorting articles, particularly "w", M h 3 letter mail, in such fashion that articles having a com- [75] Inventor: Seymour Henig, K efisifigto'fiflwdl mon characteristic, e.g., letters with the same address, 20850 are delivered as containerized group containing at 'least a predetermined threshold number of the arti- [73] Asslgnee' l gs' g fig & Company cles. Applied to systems of the type wherein a classifyj 'ing apparatus identifies each article and delivers the Flledi y 9 1971 articles to individual article retaining stations while [211 A 142 600 retaining cognizance of the characteristics and locaa 1 tions of the articles, the invention employs receptacles which are advanced past the retaining stations, articles "209/72, 209/DIG- 214/11 R having a common characteristic being discharged into [51] Int. Cl. ..B07c 3/02 j same receptacle, means being provided whereby [58] Field of Search ..209/72, DIG. 1, 73,74; any receptacle in which less than a predetermined 214/11 number of articles have been accumulated can be t t r2 1 r1sferred to storage and later retrieved and a l-, [561 Memes CM X29299 ga ttbietthliisteinia aeti na g s UNITED STATES PATENTS dition of more articles having the same characteristic.

3,184,061 5/1965 Levy ..209/72 R 3,300,066 l/l967 Henig "214/11 R Primary ExaminerRichard A. Schacher Attorney-Roylance, Abrams, Berdo and Kaul W 20 Claims, 14 Drawing Figures 6 Sheets-Sheet 1 LETTER //VPUT v 00050 STORAGE 0F //VC0MPLETELY F/LLED WENT/F), RECEPMCLES RECEPTACLE wan/wax: L L Y I j SUPPLY A/VD 7 TRANSFER 4 //VSER 7/0/v z/vro 60050 f 2 RETA/lW/VG smr/o/vs I DEL/VERY/ FIG. 1

INVENTOR SEYMOUR HEN/G Patented Sept. 11, 1973 3,757,939

6 Sheets-$heet :5

l9 INVENTOR H H SEYMOUR HEN/G FIG. 4 FIG. 5

Patented Sept. 11, 1973 6 Sheets-Sheet 4 INVENTOR SEYMOUR HEN/G Patented Sept. 11, 1973 6 Sheets-s 5 1 INVEN I SEYMOUR EN/G ,llllllll F 8 Patented Sept. 11, 1973 6 Sheets-Sheet 6 H2 M I P 70 @I-I-I :F' P 70* Q I? 96L 4O L.I JJ...I1.| DRIVE MOTOR i SYNCHRONlZER l L I l I :ICOMPUTER DEV'CE I I MAIN INTERFAcEi I I I FRAME UNIT 5 I 5 I 2 K E! I I0I I TAPE m I TRANSPORT g 2 1 LL I I I 5 DEVICE 5 I '5 .INTERFACE E l I; TAPE UNIT 8 I CONTROL I m I UNIT E 5 I D l D. Z; l i I g DEVICE NJ CORE INVENTOR I IESTORAGE INTERFACE l h, UNIT ISEYMOUR HEN/G l I 100 FIG. 13

METHOD AND APPARATUS FOR SORTING ARTICLES SUCH AS LETTERS This invention relates to sorting of articles, particularly flat relatively thin articles such as letters. The invention provides a method and apparatus whereby such articles can be sorted into groups of articles having a common characteristic, e.g., the geographic destination of a letter, in a greatly improved manner.

It is generally recognized that a number of factors, such as the expanding population and the economic requirement for increased man-hour productivity, necessitate mechanization of the handling of letter mail. Though a similar requirement applies to other are ticles, such as orders, bills, and like documents in certain business operations, mechanization of the handling of letters appears to be the most critical present need, and the invention will acordingly be described with particular reference to that application. Prior-art workers have done much work in connection with mechanized letter handling systems and have proposed apparatus which promises to be satisfactory for some of the operations involved. In particular, code .controlled letter sorters of various types have been provided which are capable of accepting an input of, e.g., letters per second, reading the envelope of each incoming letter and, in various ways, classifying the letters as to their distinctions. Automatedletter sorters presently in development are expected to handle letters at much higher ratio e.g., 100' letters per second. For complete systems employing such sorters to be successful, however, there must also be provided a rapid and dependable way to acquire the respective groups of letters from the sorter and to deliver such groups either for further processing or for dispatch to the common destination distinguishing the group. This current requirement has been characterized, for example, as calling for a system which will deliver the letters in bundles of letters 3-6 in. high, so that letter-sorters presently being developed will require a system delivering an average 75-letter bundle every 0.5-1 second. Stated that simply, the

problem of handling the output of the present code controlled letter sorters can be satisfied, for example,

by systems of the type disclosed in US. Pat. No.

3,300,066, issued Jan. 24, 1967, to Seymour Henig and Ervin S. Palasky.

The problem is complicated, however, by the fact that, in presently practical operation of code controlled letter sorters, the number of sorted letters to be found for any one destination at any given time is, e.g., about one-half that number which makes up the average bundle." Hence, a system designed simply to accept the groups of letters, "bundle the groups and deliver bundles equal to the output of the code controlled letter sorter necessarily fails, either to provide bundles" of the specified size, on the one hand, or to provide the desire rate of output, on the other hand. Further, the concept of handling free groups of letters and "bundling" each group involves a number of in herent disadvantages. One of these is that a simple group of letters cannot be diverted easily to a processing step other than bundling. Another is that "bundled" letters become bent or otherwise distorted so as to be unsuitable for further machine processing.

It is accordingly a general object of the invention to provide an apparatus and method which can accept the ing and handling apparatus and method capable of delivering the letters inthe same condition in which they are supplied.

Broadly stated, the invention is based upon the concept of delivering articles, such as letters, from a sorting apparatus into independently handleable receptacles rather than, e.g., onto a conveyor belt or the like. With articles having a common characteristic delivered tov a receptacle, that receptacle can then be recirculated in the event that the number of articles then present in the receptacle is less than the predetermined threshold value required for ultimate delivery.

In order that the manner in which the foregoing and other objects are attained according to the invention can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, which form a part of the original disclosure of this application, and wherein:

FIG. 1 is .a diagram illustrating one manner in which the method of the invention can be practiced;

FIG. 2 is a semi-diagrammatic illustration of an apparatus according to one embodiment of the invention;

' FIG. 3 is a perspective view of a receptacle employed with the apparatus of FIG. 2;

' FIGS. 4 and 5 are fragmentary sectional views taken I on lines 4-4 and 5-5, respectively, FIG. 3;

FIGS. 6 and 6A are diagrammatic views illustrating the manner in which the receptacle of FIGS. 3-5 is employed with the apparatus of FIG. 2;

FIG. 7 is a perspective view of the receptacle of FIGS. 3-5 with a second identical container applied thereto to provide a closed container;

FIG. 8 is a-front elevational view of a receptacle supply device employed in the apparatus of FIG. 2;

FIG. 9 is a top plan view of a receptacle transfer fork forming part of the apparatus of FIG. 2;

FIG. 10 is a top plan view of a transfer pusher-and associated conveyor mechanism forming part of the apparatus of FIG. 2;

FIG. 11 is a vertical sectional view of a receptacle restraining and releasing mechanism employed in the apparatus of FIG. 1;

FIG. 12 is a vertical sectional view taken on line 12- 12, FIG. 11; and

FIG. 13 is a schematic diagram of the computer operated control system employed in the apparatus of FIG. 2.

GENERAL DESCRIPTION OF THE METHOD cles collected at least equals a selected threshold value.

The letters are fed successively at high speed to a con- 3 ,75 7 ,93 9 Emir ventional automated device 1, typically an optical character reader, which identifies each letter, e. g., as to its address category, and transmits that identification for storage in the form of a digital electronic signal. Each letter is delivered from device 1 to a device 2 in which the letter is stored in an upright retaining station, an appropriate signal being developed and stored to indicate that the respective retaining station contains a letter of the particular category. Open top receptacles 3 are delivered successively from a receptacle storage device 4 onto a horizontal conveyor 5, each receptacle 3 so delivered being identified to receive letters of one particular category. The identification of receptacle 3 is also transmitted for storage as a digital electronic signal.

The retaining stations of device 2 are arranged in a horizontally extending series and the conveyor 5 is positioned to advance receptacles 3 along a horizontal path aligned below the series of retaining stations. Thus, each receptacle 3 is passed beneath all of the retaining stations in succession. Whenever a receptacle 3 is deposited on conveyor 5, the selected category for the receptacle is recorded at the end of a synchronous progression of such receptacle category codes, in the general fashion disclosed in aforementioned patent 3,300,066, so that the progress of the particular recep? tacle along the series of retaining stations is known. When a reference point on the receptacle reaches a predetermined point relative to the first retaining station containing a letter of the category to be collected in the receptacle, device 2 is operated to drop that particular letter, the letter accordingly coming to rest in a particular position in the receptacle. This operation is repeated for successive additional letters of that category until either the maximum allowable number of letters has been collected in the receptacle or the receptacle reaches the end of the series of retaining stations. As later described in detail, the successive letters are so deposited in the receptacle, and the receptacle is so constructed, that collected letters will be retained in a consistent orientation with all address surfaces facing in the same direction. If the number of letters collected in the receptacle by the time the end of the series is reached exceeds the threshhold value, the receptacle is then passed through a diverting station 6, without being diverted, and is delivered to suitable off bearing conveyor means. If, on the other hand, a number of letters less than the threshhold value has been collected when the receptacle reaches the end of the series of retaining stations, the receptacle is then diverted at station 6 and delivered to a storage means 7, the identity, content and position of the receptacle being retained in the form of stored digital information.

When device 2 again contains a sufficient quantity of letters of that particular category, the stored receptacle is automatically retrieved and delivered again to conveyor 5 so as to be advanced again along the series of retaining stations. During such advance of the receptacle along the series of retaining stations, additional letters of the same category are delivered, either until the number of letters collected in the receptacle reaches the threshold value or until, all of the letters of that category present in device 2 having been collected, the receptacle passes again to station 6 to be delivered to the off bearing means or again diverted to storage, as the case may be.

TYPICAL APPARATUS EMBODIMENT FIG. 2 illustrates a typical apparatus embodiment of the invention comprising the components 2-7 hereinbefore described, the conventional optical character reader or the like indicated at 1 in FIG. 1 being omitted for'simplicity. Device 2 can be of the type disclosed in 'U.S. Pat. No. 3,300,066 and includes a horizontally extending series of letter retaining stations 10. Each station 10, as seen in greater detail in FIGS. 6 and 6A, is in the form of an upright slot-like compartment, open at the top for insertion of letters and closed at the bottom by a pivoted door 11. Device 2 further comprises a plurality of compartmental traverse cars 12 supported for movement in the manner indicated by the arrows in FIG. 2, cars 12 being operated to receive the individually identified letters from the device 1, FIG. 1,and deliver the letters selectively to the retaining stations 10. During operation of device 2, data in the form of digital electronic signals is developed and stored in a control computer, later described with reference to FIG. 13, to correlate letter category and retaining station position information in the manner disclosed in US. Pat. No. 3,300,066.

LETTER COLLECTING RECEPTACLES I Receptacles 3 are identical, each comprising a bottom wall 15, an upwardly and rearwardly inclined trailing end wall 16, an upright front end wall 17, and upright side walls 18 and 19. The receptacle is advantageously molded as an integral piece from a suitable polymeric material. Bottom wall 15 is of transversely corrugated configuration, presenting a plurality of straight, elongated, upwardly opening notches 20 which extend transversely of the receptacle so as to be parallel to end walls 16 and 17. As best seen in FIGS. 6 and 6A, each notch 20 is of triangular cross-section, being defined by a flat rear surface and a flat front surface, with the rear surface slanting upwardly and rearwardly at a larger angle (relative to the vertical plane passing through the bottom of the notch) and the front surface slanting upwardly and forwardly at a smaller angle. Bottom wall 15 is continuous, notches 20 extending from side wall 18 to side wall 19.

Notches 20 are identical and can be considered as arranged side-by-side in a series which progresses from end wall 16 to end wall 17, with the first notch 20' of the series extending along the bottom edge of the trailing end wall 16 in such fashion that the front face of end wall 16 constitutes an extension of the rear surface of notch 20. The mouth width of the notches, i.e., the space between adjacent pairs of ridges presented by the notches, and thus the effective spacing of the notches, approximates the maximum thickness of the letters to be collected.

Rear end wall 16 has a slant height which approximates three quarters of the maximum width of the letters to be collected in the receptacle. Front end wall 17 extends upwardly at right angles to the plane of bottom wall 15 for a distance approximately one-third the projected height of wall 16 and terminates in a straight upper edge. Side wall 18 includes a flat lower portion 21 and a flat upper portion 22, the two portions 21, 22 lying in vertical planes with the plane of portion 22 displaced upwardly from that of portion 21 and the two portions being integrally joined in such fashion that the lower edge of portion 22 is defined by a downwardly facing outer shoulder 23. Similarly, side wall 19 includes lower portion 24, upper portion 25, and a downwardly facing shoulder 26 along the lower edge of portion 25. The transverse dimensions of the receptacle are such that the space between the opposing faces of portions 22 and 25 is very slightly greater than the distance between the outer face of portion 21 and the outer face of portion 24. At the front end of the receptacle, the front edges 27, 28 of upper portions 22 and 25, respectively, slant upwardly and rearwardly at the same angle of inclination as rear end wall 16, edges 27, 28 being straight and commencing at the upper edge of front wall 17.

The upper edges of side wall portions 22 and 25 are straight andparallel to the bottom wall. The upper edge of portion 22 includes a front half 29, FIGS.3 and 5, in the form of a straight elongated bead of circular transverse cross-section such that the edge portion 29 presents a right cylindrical surface extending for substantially more than 180, that surface joining inclined shoulders 30, FIG. 5. The upper edge of portion 22 also includes a rear half 31 in the nature of a groove having a curved transverse cross section which extends for more than 180 and has a radium of curvature substantially equal to that of the bead constituted by upper edge portion 29. The upper edge of upper portion 25 of side wall 19 is identical with that of portion 22 of side wall 18 and includes a front half in the formof a bead 32 and a rear half in the form of a groove 33.

RECEPTACLE CONVEYING MEANS The main receptacle conveying means 5 can, as shown FIG. 2, include a first endless belt conveyor 40 arranged to receive receptacles 3 from storage device 4 and deliver the receptacles successively to a short, driven roller conveyor 41. Conveyor 41 delivers the receptacles ,to a horizontal endless belt conveyor 42 extending below the horizontally extending series of letter retaining stations of device 2. Conveyor 42 in turn delivers to a short endless belt conveyor 43 located at station 6, and that conveyor in turn is arranged to deliver to an off-bearing conveyor 44.

Cooperating with main conveying means 5' are a first recirculating conveyor 45 which extends vertically in a location beside'the path of travel of the receptacles at station 6, and a second recirculating conveyor 46 which extends vertically beside roller conveyor 41.

. Conveyor 45 isan endless ladder conveyor including conveyor 51 or the roller conveyor 41.

A receptacle transfer device 52, FIGS. 2 and 9, is

provided at diverting station 6 and comprises a parallelogram linkage including arms 53 and a cross-link 54, arms 53 being of equal length and pivoted at one end respectively to stationary supports 55, and link 54 having its ends pivoted respectively to points on arms 53 equally spaced from the respective supports 55. A fork 56, best seen in FIG. 9, is carried by the free ends of arms 53 and comprises two parallel fingers 57 each joumalled in the free end portion of a different one of the arms 53. Fingers 57 can be cylindrical rods and include short end portions 58 which project from one side of the respective arms 53 and are rigidly interconnected by an arm 59. Long portions 60 of fingers 57 project from the opposite side of the respective arms.

The pivoted axes provided for arms 53 at supports 55 are horizontal and parallel. Arms 53 lie in a vertical plane which is parallel to and located immediately adjacent one side of the aligned assembly of conveyors 5, 43 and 44. The spacing between fingers 47 and the location of device 51 relative to the conveyors are such that, when arms 53 are swung about their pivotal axes, the arcuate path of travel of fingers 57 determined by the parallelogram linkage is such that one finger 57 passes freely through the space between the delivery end of conveyor 5 and the input end of conveyor 43, and the other finger 57 passes freely through the space betweentwo of the rollers of the conveyor 43. The space between fingers 57 is less' than the length of the bottom wall 15 of receptacle 3. The parallelogram linkage has a normal position (seen in solid lines in FIG. 2) in which the fingers, 57 are disposed below the path of travel of the receptacles 3 along conveyors 5 and 43, so that, when the linkage is in its normal position, receptacles 3 can pass along the series of conveyorsand exit via conveyor 44. The parallelogram linkage is movable to an actuated position, shown in broken lines in FIG. 2, in which fork 56 is considerably above conveyors 43, 44 and immediately in front of ladder conveyor 45. The linkage is also movable to return the fork to its normal position. To accomplish such operation, asuitable conventional electrically operated reversible drive motor (not 'shown) is operatively connected to the parallelogram linkage.

Each pair of supporting fingers 47 of conveyor 45 are spaced apart by a distance effectively smaller than the spacing between fingers 57 of fork 56. Operation of the parallelogram linkage is. so controlled that movement of the linkage to bring fork 56 to its actuated position terminates with the fingers 57 of the fork disposed between the paths of upward travel of fingers 47 of conveyor 45. Accordingly, device 52 can be operated to pick up a receptacle 3 from conveyor 43, when that receptacle has been identified as containing a number of letters less than the threshold value, transfer the receptacle 3 to the actuated position shown in broken lines in FIG. 2, and hold the receptacle there until a pair of fingers 47 of conveyor 45 is engaged beneath the receptacle, the receptacle then being carried upwardly by conveyor 45, and the parallelogram linkage then being operated to return fork 56 to its normal position.

Storage means 7 comprises a plurality of endless horizontal storage conveyors 65, FIG. 2, each comprising two relatively narrow endless belts 66 which are parallel and spaced apart, as seen in FIGS. 11 and 12. Conveyors are arranged in a vertical series and are of equal length such that the input end of each conveyor 65 is adjacent conveyor 45 and the delivery end of each conveyor 65 is adjacent conveyor 46. Bypass conveyor 51, FIG. 2, identical with storage conveyors 65 but arranged with its input end adjacent conveyor 46 and its delivery end adjacent conveyor 45, is provided below the lowermost storage conveyor 65. The arrangement of recirculating conveyors 45, 46, storage conveyors 65 and bypass conveyor 51 is such that any receptacle 3 diverted onto conveyor 45 can be delivered to any one of the storage conveyors 65 and, when desired, recovered fromthat storage conveyor, transferred to conveyor 46 and delivered by conveyor 46 to conveyor 5 to again be passed beneath the letter retaining stations 10.

To transfer receptacles 3 from conveyor 45 to storage conveyors 65, a plurality of transfer devices 70, FIG. 10, are employed, there being one device 70 for each conveyor 65. Transfer device 70 comprises an endless belt 70 carried by two horizontally spaced rollers 72 arranged in turn about vertical axes and so positioned that the straight runs of belt 71 are parallel to and spaced forwardly from the front of conveyor 45, the distance between conveyor 45 and device 70 being adequate to allow the fingers 47 of conveyor 45, and any receptacle 3 carried thereby, to pass between conveyor 45 and transfer device 70. Two pusher bars 73 are secured to belt 71 in such fashion as to project horizontally outwardly therefrom, the pusher bars 73 being at right angles to the straight runs of belt 71 when occupying positions along the straight runs. Pusher bars 73 are of a length such that their tips pass near, but do not engage, the adjacent straight runs of belts 48 of conveyor 45 when device 70 is operated. Thus, during the appropriate half of each complete cycle of movement of belt 71 in its closed path, one of the bars 73 will be caused to sweep horizontally completely across the space between transfer device 70 and conveyor 45.

The input ends of storage conveyors 65 are spaced from the vertical path of travel of the nearer ones of fingers 47 of conveyor 45 by a distance significantly less than the length of bottom wall of receptacle 3. The nearer one of rollers 72 of transfer device 70 is adjacent the input end of the corresponding one'of the storage conveyors 65. Accordingly, when a receptacle 3 is supported by one set of fingers 47 of conveyor 45 and conveyor 45 is stopped with that set of fingers aligned horizontally with the upper run of the respective storage conveyor 65, one half cycle of operation of the transfer device 70 for that storage conveyor will cause one of the pusher bars 73 to come into flush engagement with front wall 17 of the receptacle and will result in the receptacle 3 being pushed horizontally off of the fingers 47 and onto the storage conveyor 65.

To accomplish such operation, a continuously operating rotary electric motor (not shown) is operatively connected to one of the rollers 72 via an electromagnetically operated clutch-and-brake device so that the clutch can be controlled to cause the device 70 to operate for one half cycle on demand. Conveyors 45 and 46 can be 'driven in the same fashion, with the clutches operated to disconnect the drive motors, so stopping the conveyors, on demand.

One of the transfer devices 70 is also located adjacent conveyor 46 at the input end of bypass conveyor 51 and is operated, in the same manner just described, to transfer receptacles 3 from conveyor 46 to the bypass conveyor when required.

Storage conveyors are of a length adequate to accommodate a plurality of the receptacles For each storage conveyor, belts 66 are driven continuously, in a direction to carry the receptacles from conveyor 45 toward conveyor 46, and a receptacle stop device 75, shown in detail in FIGS. 11 and 12, is provided adjacent the delivery end of the storage conveyor to stop the first receptacle 3 to arrive, other receptacles 3 then accumulating behind the stopped receptacle. Each stop device comprises a horizontal frame plate 76 which underlies the upper run of belts 66 of the storage conveyor and is provided with a rectangular central opening 77 slidably accommodating a stop member 78. Member 78 is an integral block having a top face provided with spaced ridges 79, FIG. 11, which extend transversely of belts 66 and are of cross-sectional shape and size to engage in the complementary downwardly opening notches 20b presented by the bottom wall 15 of each receptacle 3. Member 78 has two flat dependent flanges 80 which are parallel with the line of travel of conveyor 65 and spaced apart transversely thereof, each flange 80 being provided with a downwardly opening vertical slot 81. Between flanges 80, member 78 has a flat bottom face 82 lying in a horizontal plane.

Member 78 is supported vertically by an eccentric roller cam 83 fixed to a horizontal shaft 84 which extends through slots 81, the surface of cam 83 being en gaged with bottom face 82. At one end, shaft 84 is connected to and supported by an electromagnetically operated half-revolution clutch 85 driven by a continuously operating rotary electric motor 86 secured to plate 76. At its other end, shaft 84 is journalled in a bracket 87 secured to and depending from plate 76.

Cam 83 is so dimensioned and oriented that one halfrevolution of shaft 83 will cause the cam to raise member 78 to interpose ridges 79 in the path of travel of the bottom wall 15 of any receptacle carried to the device 75 by the conveyor 65, and the next half-revolution of shaft 84 will cause the cam to allow member 78 tofall by gravity until ridges 79 are below belts 66 of the conveyor and thus out of the path of travel of the bottom wall of the receptacle. When member 78 has been raised by cam 83, a receptacle 3 being advanced by conveyor 65 will ratchet over ridges 79 until those ridges engage in notches 20b and the receptacle is stopped. When member 78 is lowered by the next halfrevolution of the cam, the receptacle is immediately released and carried forward by conveyor 65 travelling off the delivery end of that conveyor and onto a set of the supporting fingers 49 of conveyor 46.

Conveyors 40-44 are driven continuously at the same feed rate. Conveyors 45 and 46 are driven incrementally, so that the respective pairs of fingers 47 of conveyor 45 and the respective pairs of fingers 49 of conveyor 46 stop in alignment with the upper runs of storage conveyors 65 at the end of each increment of travel of conveyors 45,46.

EMPTY RECEPTACLE SUPPLY Since the distance between the outer faces of lower side wall'portions 21 and 24 is slightly less than the distance between the inner faces of the upper side wall portions 22 and 25, a plurality of the receptacles 3 can be arranged as a nested stack in the manner seen in FIG. 8. With the receptacles thus arranged, retaining dogs 90 of stack support 91 can be engaged respectively beneath shoulders 23 and 26 of the lowermost receptacle 3 to support the stack of receptacles. A second set of retaining dogs 92 is provided which can be brought into and out of engagement beneath the shoulders 23 and 26, respectively, of the next higher receptacle 3 in the stack. Accordingly, with dogs 92 engaged, dogs 90 can be actuated to release the lowermost receptacle, and the dogs 92 can then be disengaged to allow the stack to descend until the dogs 90, having been returned to their original positions, engage and support the newly arrived receptacle 3. The dogs 90, 92 can be operated by a suitable electromagnetic actuator (not shown) to drop the receptacles 3 one at a time onto conveyor 40, with the time of deposit of each receptacle accurately determined. Conveyor 40 is continuously driven to deliver the receptacles successively onto roller conveyor 41 which, in turn, delivers them to main conveyor 42.

ANCILLARY EQUIPMENT Receptacles 3 containing at least the threshhold number of letters at the time they reach the end of conveyor 42 are passed through station 6 by conveyor 43 and delivered onto conveyor 44. Any additional operations required can be accomplished while the recepta- COMPUTER OPERATED CONTROL SYSTEM AND ITS FUNCTION IN CARRYING OUT TI-I METI-IOD FIG. 13 illustrates a control system employed to ac-' complish operation of the various components of the apparatus of FIG. 2 in proper sequence and timed relation to carry out the method. The control system comprises a conventional general purpose electronic digital computer, indicated generally at 100 and comprising tape and core data storage and peripheral interface units to cooperate with the electromechanical actuators employed in the apparatus, as well as with the optical character reader and an input-output multiplexer. Data is supplied in the form of digital signals to the computer, via interface unit 101, with respect to, e.g.,'

letter destination address as determined by the optical reader 1, and the location of the particular letters in the retaining stations 10, this being accomplished generally in accordance with my U.S. Pat. No. 3,512,949, issued Apr. 4, i967, and in such fashion that the computer 100 maintains a running inventory of the letters of various address categories stored in the retaining stations 10, with the inventory including data representing the location of each retaining station 10 which contains a letter of a particular category. Periodically, the computer 100 operates to make a determination of the letter category which should next be unloaded from the retaining stations 10 in order to accomplish an optimum inventory reduction based on the letter content of the retaining stations 10.

Data is also stored in computer as to the time sequence of the various events necessary to determine the location and content of any receptacle 3 which has entered the apparatus In this regard, operation of conveyors 40-44 is continuous and at constant rate so that, with at least on reference point on each conveyor represented by stored data, the relative position of any point on each conveyor is determinable by the computer at any time during operation of the apparatus. Since the time of each actuation accomplished under control of the computer will be known, including, for example deposit of an empty receptacle 3 on input conveyor 40, and deposit of a partially filled receptacle I 3 on roller conveyor 41 by recirculating conveyor 46, computer 100 can determine the position and condition of all receptacles 3 which are delivered to the apparatus throughout the entire time until the receptacle is discharged onto off-bearing conveyor 44. It is to be noted particularly that, following the procedures described in my US. Pat. No. 3,312,949 the operation of the doors ll of retaining stations 10 is controlled so that letters of the I selected address category are dropped successively, with the succession proceeding in the direction of travel of receptacles 3 on conveyor 42, and in timed relation with the progress of a predetermined point or points on conveyor 42 along the path established below the series of retaining stations by the conveyor 42.

The position of each receptacle 3 on conveyor 42 is determined precisely, so that, in effect, operation of computer 100 to open doors 11 is timed in relation to 'the position on conveyor 42. of the first notch 20 of each receptacle 3. Thus, the controlling action of the computer 100 is such that, when a letter address category to be unloaded has been selected and a receptacle 3 has been placed on conveyor 42, either-from conveyor 40 or from conveyor 46, the door 11 for the first station 10 in the series which contains a letter of that category will be opened when notch 20' of the receptacle is just short of having reached precise vertical alignment below that particular retaining station 10, to allow for the very short free fall time required for the letter to drop from its retaining station to a position in which the bottom edge of the letter engages the bottom wall of the receptacle 3. Conveyor 42 is so positioned relative to the series of retaining stations 3 that each letter drops, in free fall, through a distance that is not much greater than the maximum width of the letters. Accordingly for practical purposes, the letters'retain their essentially vertical disposition, initially determined by the confining action of stations 10 and the guiding action of doors 11 during opening, throughout their free fall, and the position of the bottom edge of the dropped letter relative to notch 20' is therefore predetermined within practical limits.

The bottom edge of the letter dropped from the first selected retaining station will, therefore, come into engagement in notch 20'. Since the receptacle 3'is being advanced continuously by conveyor 42, the letter so dropped into notch 20' will pivot about its lower edge, with its upper edge swinging rearwardly, until the letter comes to rest on the trailing end wall 16 of the receptacle 3. v

Computer 100 similarly determines that the opening of door 11 of the next retaining station 10 containing a lll letter of the proper category will occur when the notch 20 next adjacent to notch 20' is just short of precise vertical alignment with that particular retaining station 10. Accordingly, the letter dropped from that retaining station will drop vertically until its bottom edge engages in that notch 20, this letter then pivoting rearwardly (as the receptacle 3 continues to advance) until it comes to rest on the first letter and is thus supported indirectly by end wall 16. This operation is continued, under control of the computer, until the receptacle 3 has advanced to the end of conveyor 42 and either all letters of the selected category which had been retained in the series of stations have been dropped into the receptacle 3 or the receptacle 3 has been filled to its capacity. in the latter event, the thecomputer is programmed to assure that no further letters of the selected category are dropped during passage of this particular receptacle 3 along the series of stations 10, the residue of letters of the selected category remaining in the stations 10 until the computer again selects that category for unloading.

A part of the data stored in computer 100 for control purposesis the predetermined threshhold value of letters to be collected in each receptacle 3, this value being the same .for all receptacles and being the reference for determining whether, when the receptacle 3 reaches the end of conveyor 42, the device 52 is to be operated to divert the receptacle to the first recirculating conveyor 45 or' is to be left unactuated until the receptacle 3 has been passed by conveyor 43 onto the off-bearing conveyor 44. Computer 100 is programmed to make a count of the letters actually dropped into each receptacle 3 and to compare that count with the predetermined threshhold value, the count being derived in the manner disclosed in my US. Pat. No. 3,312,949. When the receptacle reaches the delivery end of conveyor 42, that count is compared by the computer with the threshhold value. If the actual count is more than the threshhold value, the receptacle is simply allowed to proceed over conveyor 43 onto conveyor 44, the device 52 remaining inactive. If the count is less than the threshhold value, the computer commands operation of device 52, in timed relation to travel of the receptacle 3, so that arms 53 pivot upwardly and fingers 57 of fork 56 engage the bottom of the receptacle and lift the same to the position shown in broken lines in FIG. 2, so that the next set of fingers 47 of conveyor 45 to arrive at that position will, passing between the fingers of fork 56, engage the bottom of the receptacle 3 and cause the same to be conveyed upwardly.

The conveyor 45 thus receives only the ones of receptacles 3 which have collected from device. 2 a quantity of letters of known address category which is less than the predetermined threshhold value. I Each receptacle 3 so received by conveyor 45 is carried upwardly thereby, its position on the conveyor being known because of the known timed relation of operation of conveyor 42, diverting device 52, and conveyor 45. Also stored in computer 100 is data indicative of the state of each storage conveyor 65, i.e., whether the storage conveyor is full or can accommodate an additional receptacle 3. Accordingly, when the fingers 47 of conveyor 45 which support the receptacle 3 pause at the input end of each storage receptacle 3, the computer recognizes through its stored data whether the receptacle can be transferred to the storage conveyor. If the storage conveyor can not accommodate the receptacle, the receptacle is simply advanced by conveyor 45 to the next storage conveyor. As soon as the receptacle is brought by conveyor 45 to the input end of the first storage conveyor 65 capable of accepting a receptacle, the computer causes actuation of the corresponding transfer device to push the receptacle ofi' fingers 47 onto the input end of the storage conveyor 65. If that storage conveyor contains no receptacles, the receptacle 3 delivered thereto by device 70 advances until it is engaged and stopped by the stop device associated with that storage conveyor. If the storage conveyor already supports one or more earlierarrived receptacles, the newly arrived receptacle 3 advances until its end -wall 16, now leading because the receptacle is reversed with reference to travel, engages end wall 17 of the receptacle last placed on the storage conveyor. In either event, digital signals are stored in computer representative of the position of the particular receptacle 3 on the particular storage conveyor 65.

During operation of the apparatus, the inventory of any particular letter category in the series of retaining stations 10 varies according to new letters of that category supplied via operation of traversecars 12 and unloading of letters of that category from the retaining stations, and a continuing count of this inventory is maintained a stored data in computer 100. Assuming that letters of a particular category have been once unloaded into one or more receptacles 3 and that one receptacle 3, containing an amount of letters of that category less than the threshhold number, has been diverted to one of the storage conveyors 65, and the inventory count for that category is such that is is desirable to again unload letters of that category, the program dominating operation of computer 100 may call for unloading of the letters into one or more empty receptacles from device 4, or for recall of the stored, partially filled receptacle, or for both. Assuming the stored, partially filled receptacle is to be recalled from storage, computer 100 operates to actuate the stop device 75 of the particular storage conveyor 65 on which the receptacle is supported, the receptacles thereon being thus released and delivered directly onto respective sets of fingers 49 of the second recirculating conveyor 46. Each receptacle 3 so delivered by the storage conveyor to conveyor 46 is identified by the computer and such of the receptacles as may have been ahead of the recalled receptacle 3 on the storage conveyor 65 are delivered, by conveyor 46 and the by-pass transfer device 70, to by-pass conveyor 51 to be again delivered onto the same or a different storage conveyor 65 via conveyor 45. The recalled receptacle 3 is allowed to pass the delivery end of by-pass conveyor 51 and is deposited by conveyor 46 onto roller conveyor 41. Conveyor 41 delivers the recalled receptacle to main conveyor 42 and the receptacle is thus again passed beneath the series of retaining stations 10 to receive additional letters of the same category as those already in that receptacle.

During the pass of the receptacle under the series of stations 10, the computer operates to open the respective doors 11 in timed relation to the travel of the first unoccupied notch 20 of the receptacle, so that the first letter dropped falls into that notch, and the group of letters is then built letter by letter as earlier described. A count of the newly dropped letters is maintained by the computer and added to the count of letters already in the recalled receptacle, so that dropping of letters into that receptacle can be terminated when the total of letters therein reaches the threshhold value. If the total reaches the threshhold value, the recalled receptacle is released via conveyor 43 onto conveyor 44; if not, the receptacle can again be recycled to storage as hereinbefore described.

It is thus apparent that the combination of conveyors 40-44 diverting device 52, recirculating conveyors 45 and 46, storage conveyors 65, transfer devices 70, and stop devices 75, using the receptacles 3, can be controlled automatically by computer to cooperate with a code controlled sorter comprising devices 1 and 2 in such fashion as to overcome problems stemming from the fact that the number of articles of a given category present in the sorter may not be adequate to form a group to be delivered. The ability to recycle partially filled receptacles through storage, so that each receptacle is ultimately filled to a predetermined extent, makes it possible to capitalize more fully on the high operating speeds of which code controlled sorters, such as those shown in my U.S. Pat. No. 3,300,066, are capable.

The method and apparatus are characterized by the capability of providing faced groups of letters, that is, groups in which the address faces of all of the letters face in the same direction, and with the letters being in essentially the same condition as when delivered to the sorter, that is, with no bending, crimping or other damage having occurred during sorting, grouping and handling. Thus, the receptacles 3 not only retain the letters in the same faced relation in which they were delivered from stations but also protect the letters from damage as the receptacles are handled, this latter capability being extended through such post-sorting operations as shipping when two of the receptacles are combined to form a complete enclosure, as in FIG. 7.

While the invention has been described with reference to a system employing only a single optical character reader 1 and sorting and storing device 2, it is to be understood that a plurality of such devices can be used in serial multiplex so that the system will have an enlarged throughput capacity without duplication of the conveying, transferring, storing and control components. The present invention therefore retains all of the advantages described in my U.S. Pat. No. 2,863,574, issued December, 1958.

What is claimed is:

1. In a sorting system of the type comprising article classifying and retaining means and release means for discharging articles from the classifying and retaining means selectively in accordance with predetermined characteristics of the articles, the combination of main conveying means;

a plurality of articles receiving receptacles adapted to be conveyed by said main conveying means, said main conveying means being constructed and arranged to advance said receptacles past the article classifying and retaining means when said receptacles are disposed on said main conveying means;

off-bearing means for receiving said receptacles from said main conveying means; receptacle storage means; first recirculating conveyor means arranged to deliver said receptacles to said storage means;

selectively actuatable transfer means operative, when actuated, to transfer a receptacle from said main conveying means to said first recirculating conveyor means; control means responsive to the cumulative number of articles discharged into each of said receptacles, as the receptacles are advanced past said article classifying and retaining means, for actuating said transfer means to transfer from said main conveying means to said first recirculating conveyor means such of said receptacles as contain a cumulative number of article less than a predetermined value; and second recirculating conveyor means for retrieving receptacles from said storage means and returning the same to said main conveying means to again receive articles from the classifying and retaining means. 2. A sorting system according to claim 1 and wherein the classifying and retaining means is of the type comprising a plurality of article retaining stations arranged in a generally horizontal series and each arranged to discharge articles downwardly,

said main conveying means extending horizontally below the article retaining stations and lengthwise of the series thereof in such fashion that each of said receptacles advanced by said main conveying means will pass successively beneath the article retaining stations, the combination further including means for correlatingeach of said receptacles supplied to said main conveying means with a particular predetermined characteristic of articles present in the retaining stations; and 7 release control means for controlling operation of the release means for discharging articles from the retaining stations in such fashionthat the articles having said particular predetermined characteristic are delivered into the correlated receptacle by successive release from the respective retaining stations as the correlated receptacle is advanced along the series of retaining stations by said main conveying means.

3. A sorting system according to claim 2 and wherein the retaining stations are of a type adapted to retain relatively thin flat articles such as letters,

said receptacles have a length which is large as compared to the thickness of the flat articles and are adapted to be advanced lengthwise by said main conveying means, and

said release control means is operative to cause discharge of the flat articles from the retaining stations in a timed succession such that the first flat article discharged drops to a location adjacent the trailing end of the advancing receptacle and each successive discharged article drops to a location immediately ahead of the preceding discharged article.

4. A sorting system according to claim 3 wherein each of said receptacles is provided with a longitudinally extending series of upwardly opening locator notches each dimensioned to receive and retain the bottom edge of one of the flat articles. 5. A sorting system according to claim 2 and further comprising means for retaining a supply of said receptacles and delivering the same individually to said main conveying means. 6. A sorting system according to claim 1 and further comprising means adjacent said off-bearing means and operatively arranged to apply covers to said receptacles. 7. A sorting system according to claim 1, wherein said storage means comprises a plurality of horizontal storage conveyors arranged in a spaced group with the input ends of all of said storage conveyors disposed at one side of the group and the delivery ends of said storage conveyors disposed at the other side of the group; said first recirculating conveyor means includes a conveyor extending past the input ends of said storage conveyors; and said second recirculating conveyor means includes a conveyor which extends past the delivery ends of said storage conveyors. 8. A sorting system according to claim 7 and further comprising a plurality of receptacle transfer devices each located adjacent the input end of a different one of said storage conveyors and cooperating with said first recirculating conveyor means for selective delivery of receptacles to the respective storage conveyor. 9. A sorting system according to claim 7, and further comprising a plurality of receptacle stop devices each operatively arranged adjacent the delivery end of a different one of said storage conveyors and actuatable between an operative position, in which the stop device will engage and hold stationary a receptacle advanced thereto by thestorage conveyor, and a retracted position, in which the stop device allows receptacles to be delivered by the storage conveyor. 10. A sorting system according to claim 7 and further comprising a by-pass conveyor having an input end and a delivery end and arranged with its input end adjacent said second recirculating conveyor means at a point downstream from said storage conveyors and its delivery end adjacent said first recirculating conveyor means at a point upstream from said storage conveyor means; and selectively actuatable transfer means adjacent the input end of said by-pass conveyor for transferring receptacles from said second recirculating conveyor means to said by-pass conveyor. 11. A sorting system according to claim 10 and further comprising a stop device disposed at the delivery end of said bypass conveyor and actuatable between an operative position, in which the stop device will engage and hold stationary a receptacle advanced thereto by said by-pass conveyor, and a retracted position,

in which the stop device allows receptacles to be delivered by said by-pass conveyor to said first recirculating conveyor means.

12. In a sorting system for thin flat articles such as letter, the combination of classifying and retaining means comprising means for receiving the articles and determining a characteristic thereof, a plurality of article retaining stations arranged in a series and each adapted to retain one of the arreleased articles having the selected characteristic are deposited in the receptacle, said control means including means for determining whether the total of articles released for deposit in the receptacle reaches a predetermined threshhold value; receptacle storage means; receptacle diverting means cooperating with said conveying means in a location to be reached by the receptacles after the receptacles have been carried 3 5 past said retaining stations,

said diverting means being actuatable between a first position, inwhich receptacles are allowed to pass said diverting means, and a second position,

said control means being operative to actuate said diverting means to said second position, to divert a receptacle to said storage means, only when the receptacle contains a number of articles less than the predetermined threshhold value when the receptacle reaches said diverting means; and means operated by said control means for recalling from said storage means and delivering to said conveying means a receptacle containing articles of a particular characteristic when additional articles having that characteristic have been accumulated insaid series of retaining stations.

13. In an article sorting system adapted to deposit groups of like articles in receptacles, the combination of conveying means for conveying the receptacles to a discharge point after articles having been deposited therein;

receptacle diverting means operatively arranged relative to said conveying means and actuatable to divert therefrom receptacles which contain a number of articles which is less than a threshhold value;

receptacle storage means;

second recirculating conveyor means arranged to deliver receptacles from said storage means to said conveying means.

14. An article sorting apparatus according to claim 13, wherein said storage means comprises a plurality of continuously driven endless storage conveyors each of an effective length adequate to accommodate a plurality of the receptacles to be stored and each having an input end and a delivery end, said storage conveyors being arranged in a spaced series with the input ends of the storage conveyors all disposed at one side of the series and the delivery ends of the storage conveyors all disposed at the other side of the series; and

a plurality of stop devices each disposed adjacent the delivery end of a different one of said storage conveyors and operative selectively to engage and stop a receptacle advanced thereto by the storage conveyor and to release the conveyor for delivery by the storage conveyor.

15. An article sorting apparatus according to claim 14, wherein v each of said storage conveyors comprises twoparallel endless belts extending side by side but spaced apart, and

said stop devices are each located between said belts of the respective one of said storage conveyor.

16. In the sorting of letters by first depositing the letters individually in upright retaining'stations arranged in a horizontal series and then releasing letters of a selected category from the retaining stations in progression along the series and in timed relation with movement of a conveyor arranged below the retaining stations, the improvement comprising advancing along the conveyor an open top receptacle, the bottom of said receptacle having a series of upwardly opening locating and retaining notches, the receptacle having an end wall which trails as the receptacle is moved along the conveyor and which slants upwardly and rearwardly; and dropping the letters toward the bottom of the receptacle in such fashion that each letter is substantially vertical as it drops and the lower edge thereof engages in one of the notches presented by the bottom wall of the receptacle, the first letter dropped engaging in a notch adjacent the trailing end wall of the receptacle and continued advance of the receptacle then causing that letter to pivot into engagement with the trailing end wall, the other letters being similarly dropped to engage in successive ones of the locating and retaining notches and continued advance of the receptacle causing each letter to pivot into engagement with the next preceeding letter.

17. The method for providing a group of flat, relatively thin articles such as letters, comprising providing a plurality of the articles in a plurality of upright retaining stations with each article disposed in a different one of the stations, the stations being arranged in a series which extends at least generally horizontally;

advancing an open top receptacle beneath the retaining stations with the receptacle travelling lengthwise of the series at a predetermined constant rate, the receptacle having a bottom provided with a plurality of upwardly opening article positioning notches, the trailing end of the receptacle presenting an upwardly and rearwardly inclined article supporting surface, the first one of the article positioning notches being located immediately in advance of the bottom edge of the upwardly and rearwardly inclined article supporting surface and the remaining notches being present as a series progressing toward the leading end of the receptacle and in which the effective spacing of the notches approximates the maximum thickness of the articles; dropping a first article from one of the retaining stations when the receptacle has advanced to a position such that the first article will drop to a position in which the lower edge thereof is engaged in the first notch of the series of notches, continued advance of the receptacle causing the first article to tilt rearwardly until it is engaged with and supported by the upwardly and rearwardly inclined article supporting surface; and successively dropping additional articles from their respective retaining stations in such timed relation to the advance of the receptacle that each article is dropped to a position in which the lower edge thereof is engaged in one of the article positioning notches just in advance of the preceding article, advance of the receptacle causing each successive article, when so dropped and engaged in one of the notches, to tilt rearwardly and come to rest in an upwardly and rearwardly inclined position in which the same is supported by the immediately preceding article. 18. The method for sorting articles into groups of articles having the same characteristic, comprising depositing the articles in a plurality of article retaining stations arranged in a series; I maintaining an inventory of the articles so deposited in the retaining stations so that the number of articles of any given characteristic present in the series is known; successively passing a plurality of receptacles along the series of retaining stations; releasing articles of each characteristic from the successive retaining stations of the series into a different predetermined one of the receptacles as that receptacle is passed along the series; maintaining a count of the articles released into each receptacle; removing for further handling those of the receptacles in which the number of articles is at least equal to a predetermined threshhold value; diverting to storage those of the receptacles in which the number of articles is less than the threshhold value, the step of depositing articles in the retaining-stations continuing over a period which is long in comparison to the time required to pass a receptacle along the series of retaining stations;

recalling the stored receptacles from storage when additional articles having the characteristics of the articles already present in the respective stored receptacles have been deposited in the series of retaining stations; and

passing the recalled receptacles again along said series of retaining stations to again receive articles therefrom having the respective characteristics of the articles already present in the receptacles.

19. The method according to claim 18, wherein the articles to be sorted are letters and the charac teristics are address categories;

the letters are deposited in the retaining stations in upright position with the addressed sides of the letters all facing in the same direction throughout the senes;

the receptacles are each provided with a plurality of locator notches arranged in a series; and

the letters are each released from the stations into engagement with one of the locator notches of the respective receptacle and are retained in the receptacle with the addressed sides of the letters all facing in the same direction.

20. The method according to claim 19, wherein the series of retaining stations extends horizontally;

the receptacles are open top receptacles and are passed along a horizontal path immediately below and aligned with the series of retaining stations; and

the letters are dropped from the retaining stations into the respective receptacles.

II! l 

1. In a sorting system of the type comprising article classifying and retaining means and release means for discharging articles from the classifying and retaining means selectively in accordance with predetermined characteristics of the articles, the combination of main conveying means; a plurality of articles receiving receptacles adapted to be conveyed by said main conveying means, said main conveying means being constructed and arranged to advance said receptacles past the article classifying and retaining means when said receptacles are disposed on said main conveying means; off-bearing means for receiving said receptacles from said main conveying means; receptacle storage means; first recirculating conveyor means arranged to deliver said receptacles to said storage means; selectively actuatable transfer means operative, when actuated, to transfer a receptacle from said main conveying means to said first recirculating conveyor means; control means responsive to the cumulative number of articles discharged into each of said receptacles, as the receptacles are advanced past said article classifying and retaining means, for actuating said transfer means to transfer from said main conveying means to said first recirculating conveyor means such of said receptacles as contain a cumulative number of article less than a predetermined value; and second recirculating conveyor means for retrieving receptacles from said storage means and returning the same to said main conveying means to again receive articles from the classifying and retaining means.
 2. A sorting system according to claim 1 and wherein the classifying and retaining means is of the type comprising a plurality of article retaining stations arranged in a generally horizontal series and each arranged to discharge articles downwardly, said main conveying means extending horizontally below the article retaining stations and lengthwise of the series thereof in such fashion that each of said receptacles advanced by said main conveying means will pass successively beneath the article retaining stations, the combination further including means for correlating each of said receptacles supplied to said main conveying means with a particular predetermined characteristic of articles present in the retaining stations; and release control means for controlling operation of the release means for discharging articles from the retaining stations in such fashion that the articles having said particular predetermined characteristic are delivered into the correlated receptacle by successive release from the respective retaining stations as the correlated receptacle is advanced along the series of retaining stations by said main conveying means.
 3. A sorting system according to claim 2 and wherein the retaining stations are of a type adapted to retain relatively thin flat articles such as letters, said receptacles have a length which is large as compared to the thickness of the flat articles and are adapted to Be advanced lengthwise by said main conveying means, and said release control means is operative to cause discharge of the flat articles from the retaining stations in a timed succession such that the first flat article discharged drops to a location adjacent the trailing end of the advancing receptacle and each successive discharged article drops to a location immediately ahead of the preceding discharged article.
 4. A sorting system according to claim 3 wherein each of said receptacles is provided with a longitudinally extending series of upwardly opening locator notches each dimensioned to receive and retain the bottom edge of one of the flat articles.
 5. A sorting system according to claim 2 and further comprising means for retaining a supply of said receptacles and delivering the same individually to said main conveying means.
 6. A sorting system according to claim 1 and further comprising means adjacent said off-bearing means and operatively arranged to apply covers to said receptacles.
 7. A sorting system according to claim 1, wherein said storage means comprises a plurality of horizontal storage conveyors arranged in a spaced group with the input ends of all of said storage conveyors disposed at one side of the group and the delivery ends of said storage conveyors disposed at the other side of the group; said first recirculating conveyor means includes a conveyor extending past the input ends of said storage conveyors; and said second recirculating conveyor means includes a conveyor which extends past the delivery ends of said storage conveyors.
 8. A sorting system according to claim 7 and further comprising a plurality of receptacle transfer devices each located adjacent the input end of a different one of said storage conveyors and cooperating with said first recirculating conveyor means for selective delivery of receptacles to the respective storage conveyor.
 9. A sorting system according to claim 7, and further comprising a plurality of receptacle stop devices each operatively arranged adjacent the delivery end of a different one of said storage conveyors and actuatable between an operative position, in which the stop device will engage and hold stationary a receptacle advanced thereto by the storage conveyor, and a retracted position, in which the stop device allows receptacles to be delivered by the storage conveyor.
 10. A sorting system according to claim 7 and further comprising a by-pass conveyor having an input end and a delivery end and arranged with its input end adjacent said second recirculating conveyor means at a point downstream from said storage conveyors and its delivery end adjacent said first recirculating conveyor means at a point upstream from said storage conveyor means; and selectively actuatable transfer means adjacent the input end of said by-pass conveyor for transferring receptacles from said second recirculating conveyor means to said by-pass conveyor.
 11. A sorting system according to claim 10 and further comprising a stop device disposed at the delivery end of said by-pass conveyor and actuatable between an operative position, in which the stop device will engage and hold stationary a receptacle advanced thereto by said by-pass conveyor, and a retracted position, in which the stop device allows receptacles to be delivered by said by-pass conveyor to said first recirculating conveyor means.
 12. In a sorting system for thin flat articles such as letter, the combination of classifying and retaining means comprising means for receiving the articles and determining a characteristic thereof, a plurality of article retaining stations arranged in a series and each adapted to retain one of the articles and operable to release the retained article, means for delivering articles from said first-mentioned means each to a different one of said retaining stations; conveying means arranged to carry a plurality of article receiving reCeptacles past said series of retaining stations; computer operated control means operative to release from said retaining stations articles having a selected characteristic, with such release effected in timed relation with the movement of a receptacle on said conveying means such that the released articles having the selected characteristic are deposited in the receptacle, said control means including means for determining whether the total of articles released for deposit in the receptacle reaches a predetermined threshhold value; receptacle storage means; receptacle diverting means cooperating with said conveying means in a location to be reached by the receptacles after the receptacles have been carried past said retaining stations, said diverting means being actuatable between a first position, in which receptacles are allowed to pass said diverting means, and a second position, said control means being operative to actuate said diverting means to said second position, to divert a receptacle to said storage means, only when the receptacle contains a number of articles less than the predetermined threshhold value when the receptacle reaches said diverting means; and means operated by said control means for recalling from said storage means and delivering to said conveying means a receptacle containing articles of a particular characteristic when additional articles having that characteristic have been accumulated in said series of retaining stations.
 13. In an article sorting system adapted to deposit groups of like articles in receptacles, the combination of conveying means for conveying the receptacles to a discharge point after articles having been deposited therein; receptacle diverting means operatively arranged relative to said conveying means and actuatable to divert therefrom receptacles which contain a number of articles which is less than a threshhold value; receptacle storage means; first recirculating conveyor means arranged to deliver receptacles from said diverting means to said storage means; and second recirculating conveyor means arranged to deliver receptacles from said storage means to said conveying means.
 14. An article sorting apparatus according to claim 13, wherein said storage means comprises a plurality of continuously driven endless storage conveyors each of an effective length adequate to accommodate a plurality of the receptacles to be stored and each having an input end and a delivery end, said storage conveyors being arranged in a spaced series with the input ends of the storage conveyors all disposed at one side of the series and the delivery ends of the storage conveyors all disposed at the other side of the series; and a plurality of stop devices each disposed adjacent the delivery end of a different one of said storage conveyors and operative selectively to engage and stop a receptacle advanced thereto by the storage conveyor and to release the conveyor for delivery by the storage conveyor.
 15. An article sorting apparatus according to claim 14, wherein each of said storage conveyors comprises two parallel endless belts extending side by side but spaced apart, and said stop devices are each located between said belts of the respective one of said storage conveyor.
 16. In the sorting of letters by first depositing the letters individually in upright retaining stations arranged in a horizontal series and then releasing letters of a selected category from the retaining stations in progression along the series and in timed relation with movement of a conveyor arranged below the retaining stations, the improvement comprising advancing along the conveyor an open top receptacle, the bottom of said receptacle having a series of upwardly opening locating and retaining notches, the receptacle having an end wall which trails as the receptacle is moved along the conveyor and which slants upwardly and rearwardly; and dropPing the letters toward the bottom of the receptacle in such fashion that each letter is substantially vertical as it drops and the lower edge thereof engages in one of the notches presented by the bottom wall of the receptacle, the first letter dropped engaging in a notch adjacent the trailing end wall of the receptacle and continued advance of the receptacle then causing that letter to pivot into engagement with the trailing end wall, the other letters being similarly dropped to engage in successive ones of the locating and retaining notches and continued advance of the receptacle causing each letter to pivot into engagement with the next preceeding letter.
 17. The method for providing a group of flat, relatively thin articles such as letters, comprising providing a plurality of the articles in a plurality of upright retaining stations with each article disposed in a different one of the stations, the stations being arranged in a series which extends at least generally horizontally; advancing an open top receptacle beneath the retaining stations with the receptacle travelling lengthwise of the series at a predetermined constant rate, the receptacle having a bottom provided with a plurality of upwardly opening article positioning notches, the trailing end of the receptacle presenting an upwardly and rearwardly inclined article supporting surface, the first one of the article positioning notches being located immediately in advance of the bottom edge of the upwardly and rearwardly inclined article supporting surface and the remaining notches being present as a series progressing toward the leading end of the receptacle and in which the effective spacing of the notches approximates the maximum thickness of the articles; dropping a first article from one of the retaining stations when the receptacle has advanced to a position such that the first article will drop to a position in which the lower edge thereof is engaged in the first notch of the series of notches, continued advance of the receptacle causing the first article to tilt rearwardly until it is engaged with and supported by the upwardly and rearwardly inclined article supporting surface; and successively dropping additional articles from their respective retaining stations in such timed relation to the advance of the receptacle that each article is dropped to a position in which the lower edge thereof is engaged in one of the article positioning notches just in advance of the preceding article, advance of the receptacle causing each successive article, when so dropped and engaged in one of the notches, to tilt rearwardly and come to rest in an upwardly and rearwardly inclined position in which the same is supported by the immediately preceding article.
 18. The method for sorting articles into groups of articles having the same characteristic, comprising depositing the articles in a plurality of article retaining stations arranged in a series; maintaining an inventory of the articles so deposited in the retaining stations so that the number of articles of any given characteristic present in the series is known; successively passing a plurality of receptacles along the series of retaining stations; releasing articles of each characteristic from the successive retaining stations of the series into a different predetermined one of the receptacles as that receptacle is passed along the series; maintaining a count of the articles released into each receptacle; removing for further handling those of the receptacles in which the number of articles is at least equal to a predetermined threshhold value; diverting to storage those of the receptacles in which the number of articles is less than the threshhold value, the step of depositing articles in the retaining stations continuing over a period which is long in comparison to the time required to pass a receptacle along the series of retaining stations; recalling the Stored receptacles from storage when additional articles having the characteristics of the articles already present in the respective stored receptacles have been deposited in the series of retaining stations; and passing the recalled receptacles again along said series of retaining stations to again receive articles therefrom having the respective characteristics of the articles already present in the receptacles.
 19. The method according to claim 18, wherein the articles to be sorted are letters and the characteristics are address categories; the letters are deposited in the retaining stations in upright position with the addressed sides of the letters all facing in the same direction throughout the series; the receptacles are each provided with a plurality of locator notches arranged in a series; and the letters are each released from the stations into engagement with one of the locator notches of the respective receptacle and are retained in the receptacle with the addressed sides of the letters all facing in the same direction.
 20. The method according to claim 19, wherein the series of retaining stations extends horizontally; the receptacles are open top receptacles and are passed along a horizontal path immediately below and aligned with the series of retaining stations; and the letters are dropped from the retaining stations into the respective receptacles. 